1. Field of the Invention
The present invention relates to a submersible motor pump having a cooling mechanism for a motor.
The present invention also relates to a motor pump for delivering a liquid.
The present invention further relates to a tandem mechanical seal for use in a submersible motor pump.
2. Description of the Related Art
A submersible motor pump is widely used for delivering a liquid, such as sewage, wastewater, or river water, which contains debris and dirt therein. Typically, a motor is disposed above an impeller. Accordingly, under low water level conditions, the pump is operated with the motor exposed in the atmosphere. In order to cool the motor sufficiently even in such a situation, a water jacket is provided around the motor and a liquid circulates through the water jacket to thereby cool the motor.
Liquids for use in cooling of the motor include a handled liquid of the pump (i.e., a liquid to be conveyed by the pump) and a coolant dedicated for the cooling purpose. In the case of using the handled liquid of the pump, the dirt and debris can accumulate in the water jacket or cause clogging of the water jacket. As a result, the need for frequent maintenance may arise. Therefore, there has been an increasing demand for the water jacket using the dedicated coolant.
In the case of using the coolant (or cooling liquid), it is necessary to install a mechanism for circulating the coolant, in addition to a main impeller for delivering the handled liquid. As such a circulating mechanism, there has been proposed an impeller, which is provided on a rotational shaft separately from the main impeller, for circulating the coolant. The coolant should be isolated sufficiently from the motor and the handled liquid. Further, the motor should also be separated from the handled liquid. A tandem mechanical seal, which has two mechanical seals arranged in series, is conventionally used as a seal mechanism for separating the motor from the handled liquid. It has also been proposed to provide an impeller of the circulating mechanism between the two mechanical seals. However, the tandem mechanical seal, containing the impeller therein, has a complex structure. In particular, when using a centrifugal impeller as the impeller for circulating the coolant, it is necessary to devise structures for assembly.
Further, in the motor cooling mechanism using the coolant, it is necessary to provide a mechanism for dissipating heat, which has been transferred from the motor, into the exterior of a circulation passage of the coolant. One of the proposed solutions is to dissipate the heat of the coolant by heat exchange between the coolant and the handled liquid through a pump casing. However, a space between the motor and the pump casing is limited and therefore it is difficult to secure a sufficient heat-transfer area for the heat exchange. Further, air pocket (i.e., trapped air) is likely to be created in a housing space of the main impeller (e.g., in a region above the main impeller, in particular in a region behind the main impeller). Such air pocket can hinder the heat exchange between the coolant and the handled liquid. Further, the air pocket also hinders lubrication and cooling of the mechanical seal. As a result, a lifetime of the mechanical seal could be shortened.